In general, an avalanche photodiode (APD) is a photodiode having rapid speed and high response characteristics, and is an element that internally generates an electrical signal by adjusting amplitude ratio according to a change in backward voltage.
In particular, the avalanche photodiode used in an optical medical device is packaged in a TO can form, and is used to measure characteristics of light when the light output from a laser diode passes through a portion of a tissue of the human body and is introduced into the avalanche photodiode.
Most of the light is lost due to absorption and scattering of light while passing through the human body so that only a very small amount of light is introduced into the avalanche photodiode, and an avalanche photodiode that mostly uses a reverse bias voltage of not less than 100 V is used to achieve an avalanche photodiode having a high light receiving gain.
However, because the avalanche photodiode has characteristics in which a voltage gain is changed according to a backward voltage value, a high amplitude ratio is achieved by increasing a backward voltage value.
However, because the avalanche photodiode has an allowable maximum voltage value, voltage amplitude radio increases geometrically if voltage value exceeds the limit value, so that the avalanche photodiode is burned out.
Further, a voltage gain of the avalanche photodiode is changed according to temperature.
The reason why amplitude ratio is changed according to temperature is that the temperature increases as the lattice structure of the avalanche photodiode vibrates, and then the number of carriers increases so that energy level is moved by an ionization phenomenon.
Due to this phenomenon, amplitude ratio changes according to temperature even at the same backward voltage, and response characteristics of the avalanche photodiode are changed as the amplitude ratio changes so that measurement time changes even when the same amount of light is introduced into the avalanche photodiode.
When the avalanche photodiode having the characteristics makes direct contact with the human body to measure scattering light, light receiving gain also rapidly changes as operation temperature rapidly changes due to body temperature as a result of the contact.
Further, even when the avalanche photodiode does not make direct contact, the temperature of the avalanche photodiode changes due to a peripheral circuit, a voltage supply unit, or the like.
This mainly causes a measurement error of a medical device that uses a laser diode.
According to the related art, in order to overcome the problems, a technology of constantly maintaining the temperature of an avalanche photodiode by employing a thermoelectric cooler (TEC) outside an avalanche photodiode TO-can.
FIG. 1 is a perspective view of an external appearance of a laser diode measuring apparatus using a thermoelectric cooler according to the related art, and includes an avalanche photodiode 10, a thermoelectric cooler 20, a heat sink, and a cooling fan 30.
The laser diode measuring apparatus using the thermoelectric cooler 20 has heat emitting and cooling functions through a forward current and a backward current, and constantly maintains amplitude ratio by preventing a temperature change of the avalanche photodiode 10 by controlling temperature through an external control unit.
In order to use the thermoelectric cooler 20, a thermistor for detecting the temperature of an avalanche photodiode 10 and a current supply circuit for driving an thermoelectric cooler 20 should be inevitable included.
In addition to an increase of power consumption and costs, as illustrated in FIG. 1, there is a limit in that the size of the measurement apparatus is very large.
Accordingly, in an optical diagnosis system using an avalanche photodiode 10, in particular, in a structure in which scattering light should be measured by bringing the avalanche photodiode into direct contact with a surface of skin, because the size of the measurement apparatus should be large, there are many problems in measuring a severely curved portion, for example, in measuring a thyroid gland of a neck, a sentinel lymph node, or the like.